Protein films can theoretically be made from any protein which can form a gel. The process involves a transition from low protein concentration to high protein concentration.
Gel is the general name for a jellylike material which is a dilute solution of crosslinked macromolecules. Fibrin clot is the specialized name for a protein gel which is formed by the reaction in solution of fibrinogen and a specific proteolytic enzyme, usually thrombin. Other proteolytic enzymes such as reptilase or ancrod produce clots of somewhat different properties. Depending on the pH and ionic strength of the reaction solution, a coarse fibrin clot, an intermediate fibrin clot, or a fine fibrin clot will be formed. J. D. Ferry and P. R. Morrison, J. Amer. Chem. Soc., 69, 400 (1947). Other protein gels can be formed by cooling solutions of gelatin or by heat denaturation of proteins from the class including, but not limited to, serum albumin, insulin, ribonuclease, or alpha-chymotrypsin. A. H. Clark, F. J. Fudge, J. B. Richards, J. M. Stubbs, A. Suggett, Int. J. Protein Res. 17, 380-392 (1982). Gel formation of all types is well known in the art.
Some early protein films were coarse fibrin films developed in the 1940's. J. D. Ferry and P. R. Morrison, J. Clin, Invest. 23, 566 (1944). These films were made from coarse clots of fibrin which clots were made by the reaction in solution of fibrinogen with the proteolytic enzyme thrombin. Coarse clots consist of thick bundles of fibers having average bundle diameters up to approximately 3000 Angstroms. The coarse clots have void space in them such that excess solvent exists in the empty spaces between the bundles. Conventionally, protein film is made from the coarse clot by using pressure to drive out excess solvent from the clot. When pressure is applied to the coarse clot the solvent runs out through channels in the clot. The structural integrity of the thick fiber bundles is generally maintained in the resulting film.
Films made from coarse clots are useful because the natural structure of the protein fibers is preserved and the fibrous elements are arranged to lie primarily in or parallel to the plane of the film though oriented at random within that plane. Film made from coarse clots can be stretched 100 percent with subsequent elastic recovery after release of tension. The film has been used in surgical applications. Films made from coarse clots could also be used in the treatment of burns. Because coarse fibrin film is a native protein with no chemical modification or denaturation, implants are gradually absorbed with no tissue reactions. O. T. Bailey and F. D. Ingraham, J. Clin. Invest., 23, 597 (1944).
Another potential use of coarse fibrin films is for controlled release of therapeutic agents such as drugs. However, because of the great amount of void space inherent in coarse films, drug delivery can not be controlled very easily. Drugs migrate through channels in the coarse films, just as solvent runs out under pressure, rather than being released gradually as the coarse fibrin film is absorbed by the body. Modified coarse fibrin films have been used for controlled drug delivery in the eye. S. Miyazaki, K. Ishi, and M. Takada, Chem. Pharm. Bull., 30, 3405-3407 (1982). However, in the technique described in this reference, fibrin film was dried and then ground up and the mixture was compressed at 160.degree. to 170.degree. C. under high pressure. This procedure destroys the integrity of the natural fibrous network and undoubtedly produces substantial chemical modification.
Protein films made from fine or intermediate fibrin clots or made from gelatin or heat-denatured protein gels would have uses similar to the uses of coarse fibrin film. However, attempts to make protein film from either fine or intermediate fibrin clots or from gelatin or heat-denatured protein gels by the conventional pressure method proved unsuccessful. Fine fibrin clots consist of thin bundles of fiber having bundle diameters of less than approximately 200 Angstroms. The solvent in fine fibrin clots is more uniformly distributed among the fine fibers rather than existing in void spaces between the bundles, and cannot be drained by applying pressure. Attempts to force water out by exerting pressure on fine or intermediate fibrin clots destroyed the structural integrity of the fibers themselves. Pressure exerted on gels of gelatin or heat-denatured proteins similarly broke the gels up.